Colour in Fruit Juices – Carotenoids
The colour and appearance of a fruit juice is an important feature of its anticipated organoleptic quality. Generally, the preference will always be for a bright, fresh looking colour devoid of darkening or browning. Colour in fruit juices arises from the presence of water or oil soluble pigments. In rare cases both classes of pigments can be present, where perhaps the best known example is of Blood Oranges. The water soluble colours occur as Anthocyanins or Betalains while the oil soluble ones are Carotenoids.
Colours in plants exist primarily for photosynthesis and for attracting pollinating insects. The green pigment chlorophyll that occurs in all plants is the principal agent of photosynthesis
This article deals with a third class natural colours, the oil soluble Carotenoids which occur in both fruits and vegetables and are amongst the most widely distributed pigments in plants.
Carotenoids are synthesized by many plants, (and some aphids, spider mites, algae, photosynthetic bacteria, fungi and yeasts), but not by animals. Carotenoids provide colours ranging from yellow to orange to dark red. While Carotenoids are dominant in 15 – 30% of tree species, they are not found in the leaves until the end of summer when they develop. For deciduous trees, carotenoids contribute to the brilliant autumn display of yellow and orange colours. (See also Colours in Fruit Juices (1) – Anthocyanins).
Carotenoid pigments are found in photosynthetic tissues, with the highest concentration of Carotenoids being in the leaves of most green plants. The light energy absorption drives photosynthesis while also protecting chlorophyll from oxidative damage.
As neither humans nor animals can synthesise Carotenoids, they obtain them through dietary ingestion. When in circulation, Carotenoids bind to lipoproteins and enter tissues such as liver, macula, lung, adipose, brain, prostate and skin.
Carotenoids comprise of two categories, Carotenes and Xanthophylls. The Carotenes are pure hydrocarbons and generally of red colour, while the Xanthophylls contain one or more Oxygen component and are yellow in character. There are now over 1200 identified Carotenoids, some of the more commonly occurring being:-
(see PDF for image)
Carotenoids absorb light generally in the 400- 540nm (violet to green) range, resulting in the observed colours of yellow to orange to red. The long chain conjugated double bonds of all Carotenoids are responsible for colour, which is in turn affected by modifications to the basic molecular structure.
See Appendix 1 for further examples and general structures.
Carotenoids are found in many yellow and orange coloured fruits and vegetables as well as dark green vegetables, where the carotenoid colour is masked by the chlorophyll. Despite being lipophilic, carotenoids frequently occur in highly aqueous systems such as fruit and vegetable juices as dispersions or associated with proteins. In fruits, the concentration of Carotenoids is often higher in the peel than in the flesh, for example in oranges Carotenoids protect photosynthetic reactions against damage from a combination of light and oxygen.
Carotenes derive their name from the Latin or Greek names for carrot, carota or karotan respectively. Carrots contain one of the highest levels of β-Carotene itself. Carotene was discovered around 1831 but its structure was not fully elucidated until 1930-31.
The Xanthophylls are so called from the Greek xanthos for yellow and phyllon for leaves and were discovered as a yellow band in the chromatography of leaf pigments.
To continue reading with full illustrations and appendices, please download the PDF in the link below.
Download Colour in Fruit Juices – Carotenoids